The present invention is directed toward a process for producing blends of syndiotactic 1,2-polybutadiene and rubbery elastomers. The present invention is also directed to a polymer composition comprising a blend of syndiotactic 1,2-polybutadiene and a terpolymer polymerized from ethylene, at least one α-olefin monomer, and at least one diene monomer.
Tire sidewalls protect the ply and are therefore preferably resistant to weathering, ozone, abrasion, and tearing, while providing excellent flex fatigue resistance. Typical tire sidewall formulations include natural rubber (NR), styrene-butadiene (SBR), butadiene (BR), and halogenated butyl (HIIR). Ethylene-propylene-diene terpolymer (EPDM) is attractive because of its resistance to weathering and ozone.
EPDM, however, is not compatible with butadiene rubber and fillers, and has poor cut growth resistance. Blends of EPDM with crystalline polymers have shown improved cut growth properties at room temperature. At elevated temperatures, however, these materials have poor cut growth properties.
Syndiotactic 1,2-polybutadiene (sPB) is a crystalline thermoplastic resin that has a stereoregular structure in which the side-chain vinyl groups are located alternately on the opposite sides in relation to the polymeric main chain. sPB uniquely exhibits the properties of both plastics and rubber, and therefore it has many uses. It can also be blended into and co cured with natural and synthetic rubbers.
Syndiotactic 1,2-polybutadiene can be made by solution, emulsion, or suspension polymerization. Generally, syndiotactic 1,2-polybutadiene has a melting temperature within the range of about 195° C. to about 215° C., but due to processability considerations, it is generally desirable for syndiotactic 1,2 polybutadiene to have a melting temperature of less than about 195° C.
Because syndiotactic 1,2-polybutadiene is insoluble in common solvents at normal polymerization temperatures, a common technical difficulty in the synthesis of syndiotactic 1,2-polybutadiene is that the polymerization mixture is an extremely thick slurry at the commercially desirable polymer concentration of 10% to 25% by weight. This thick slurry becomes difficult to stir and transfer, thereby diminishing heat transfer efficiency and interfering with proper process control. Also, the slurry contributes to reactor fouling due to the undesirable build-up of insoluble polymer on the baffles, agitator blades, agitator shafts, and walls of the polymerization reactor. It is therefore necessary to clean the reactor on a regular basis, which results in frequent shutdowns of continuous processes and serious limitations of the run length of batch processes. The task of cleaning the fouled reactor is generally difficult and time-consuming. All of these drawbacks detrimentally affect productivity and the cost of operation.
The physical properties of rubbery elastomers can be improved by blending crystalline polymers therein. For example, incorporating syndiotactic 1,2-polybutadiene into rubber compositions that are utilized in the supporting carcass of tires greatly improves the green strength of those compositions. Also, incorporating syndiotactic 1,2-polybutadiene into tire tread compositions can reduce heat build-up and improve wear characteristics of tires. The green strength of synthetic rubbers such as cis-1,4-polybutadiene can also be improved by incorporating a small amount of syndiotactic 1,2-polybutadiene.
Blends of crystalline polymers and rubbery elastomers are typically prepared by standard mixing techniques. For example, these blends can be prepared by mixing or kneading and heat-treating a crystalline polymer and a rubbery elastomer by utilizing generally known mixing equipment such as a Banbury mixer, a Brabender mixer, an extruder, a kneader, or a mill mixer. These high-temperature mixing procedures, however, have certain drawbacks including high processing costs, polymer degradation and crosslinking, inadequate mixing, as well as various process limitations. Due to the high vinyl content of syndiotactic 1,2-polybutadiene, polymer degradation and crosslinking is a particularly severe problem for mixing syndiotactic 1,2-polybutadiene with elastomers at high temperatures.
Attempts to polymerize 1,3-butadiene into syndiotactic 1,2-polybutadiene within a rubber cement have been hampered by catalyst inefficiencies and toxicities. For example, U.S. Pat. No. 4,379,889 teaches polymerizing 1,3-butadiene into syndiotactic 1,2-polybutadiene within a rubber cement by using a catalyst system comprising a cobalt compound, a dialkylaluminum halide, carbon disulfide, and an electron donative compound. And, U.S. Pat. No. 5,283,294 teaches a similar process that employs a catalyst system comprising a cobalt compound, an organoaluminum compound, and carbon disulfide. These methods, however, are inferior because the catalyst systems that are employed suffer from low catalytic activity, poor stereoselectivity, the need for toxic, halogenated solvents, and the many drawbacks associated with carbon disulfide including low flash point, obnoxious smell, high volatility and toxicity.
Therefore, it would be advantageous to develop a new and significantly improved process for producing blends of syndiotactic 1,2-polybutadiene and rubbery elastomers.